The present invention relates to a semiconductor device having a heat dissipating metal layer on the back side thereof and, more particularly to a method of manufacturing the semiconductor device wherein a semiconductor wafer is divided into chips by laser cutting.
A semiconductor device that must have lower thermal resistance such as high-output FET used in microwave communication or the like generally requires reduced thickness of the semiconductor substrate, for example, 50 .mu.m or less. It is also necessary to form a thick heat dissipating metal layer on the back-side of the substrate in order to improve the heat dissipation efficiency, and to prevent the substrate from breaking during handling.
In using a dicing method to divide such a semiconductor wafer having the dissipating metal layer into semiconductor chips, which is employed for ordinary semiconductor wafers, clogging of the dicing saw may occur due to the heat dissipating metal sticking thereto. In using a wet etching method instead of the dicing method to divide the wafer, separated semiconductor chips tend to be scattered in a solvent bath at the end of the separation process. The chip scattering makes it time-consuming to collect the separated chips and makes a problem of chip breakage during collection.
Kosaki et al. PCT Application No. PCT/JP96/02758 discloses a method of cutting off a semiconductor wafer into semiconductor chips by means of etching and laser cutting. This method will be described below with reference to the accompanying drawings.
FIG. 5A to FIG. 5G are cross sectional views showing a flow sheet for manufacturing a semiconductor device having a heat dissipating metal layer. First, front separation grooves 3 are formed by etching using a resist pattern 2 as a mask on the surface of a GaAs substrate 1 whereon semiconductor elements have been formed (FIG. 5A). Then a first linkage metal layer 4 is formed in the front separation grooves 3 by plating or other methods (FIG. 5B).
Then wax 5 is applied to the front surface of the GaAs substrate 1, which is then bonded onto a support substrate 6 such as a glass plate or a sapphire plate. Then the back side of the GaAs substrate 1 is polished until the thickness of the GaAs substrate 1 is reduced to about 20 to 30 .mu.m (FIG. 5C). The depth of the front separation groove 3 is typically one-half or less of the thickness of the polished substrate.
A resist pattern 7, which has apertures at positions facing bottom surfaces 3b of the front separation grooves 3, is formed on the back side of the GaAs substrate 1. Using the resist pattern 7 as a mask, back side of the GaAs substrate 1 is etched until the bottom surfaces of the linkage metal layer 4 in the front separation grooves is exposed, thereby to form back separation grooves 8 (FIG. 5D). Width of the bottom surfaces 3b of the front separation grooves 3 must be greater than the width of the bottom surfaces 8b of the back separation grooves 8. In case the bottom surfaces 8b are wider than the bottom surfaces 3b, variations in the amount of etching may lead to over etching where the edge of the bottom surface 8c is excessively etched along the first linkage metal layer 4 (FIG. 6).
After removing the resist pattern 7, a plated feeder layer (not shown) is formed over the entire back surface of the GaAs substrate 1. Then a second linkage metal layer 9 is formed by plating in the back separation grooves 8 for reinforcement of the first linkage metal layer 4. This is followed by the formation of a heat dissipating metal layer 10 by electroplating on portions other than that opposing the bottoms of the front separation grooves 3 (FIG. 5E).
Then the GaAs substrate 1 is pulled off the support substrate 6 and an expand film (not shown) is attached to the heat dissipating metal layer 10. The first and the second linkage metal layers 4, 9 are cut by means of YAG laser or the like from the side of the first linkage metal layer, thereby separating the GaAs substrate 1 into semiconductor chips 30 (FIG. 5F). The linkage metal layers which have been cut off by the laser have a flange-shaped protrusion 11 with a rounded edge 27.
The semiconductor chip thus produced is bonded onto a package 12 by soldering or the like. Then wires 13 are bonded onto bonding pads located on the surface of the GaAs substrate 1. Last, the entire semiconductor chip is sealed with a resin.
According to the method of cutting the semiconductor chips by laser light, there occurs no problem like clogging of the dicing saw as experienced in the dicing method. Further the semiconductor chips which have been cut by the laser light are arranged orderly on the expand film. Thus there is no need to collect the chips, which is required in the case of using the wet etching method.
With the laser cutting method described above, however, top edges of the first linkage metal layer 4 are located in the same plane as the surface of the GaAs substrate 1. Moreover, the edges of the first linkage metal layer 4 may protrude beyond the surface of the GaAs substrate 1, because the metal formation process based on plating technique or the like. This may cause such a problem that the wires 13 and the linkage metal layer 4 are short-circuited during the wire-bonding processes. Thus it is difficult to stabilize the production yield of the semiconductor device. Moreover, when the device is heated, the wire 13 may be deformed and touch the first linkage metal layer due to thermal deformation of the resin around the wire, this results in poor reliability of the semiconductor device.
The first linkage metal layer 4 and the second linkage metal layer 9 have flange-shaped protrusions 11 after being cut by the laser light. Since the flange-shaped protrusions 11 protrude over a significant length compared to the thickness thereof and the length of the protrusions varies significantly, the protrusions 11 tend to bend during handling the semiconductor chips. The bent protrusions 11 may contact the wires and cause failures of the device.